As the development of biometric devices for identity verification, and in particular of fingerprint sensing devices, has lead to devices which are made smaller, cheaper and more energy efficient, the possible applications for such devices are increasing.
In particular fingerprint sensing has been adopted more and more in, for example, consumer electronic devices, due to small form factor, relatively beneficial cost/performance factor and high user acceptance (in particular in comparison to retina scanning etc).
Capacitive fingerprint sensing devices, built based on CMOS technology for providing the fingerprint sensing elements and auxiliary logic circuitry, are increasingly popular as such sensing devices can be made both small and energy efficient while being able to identify a fingerprint with high accuracy. Thereby, capacitive fingerprint sensors are advantageously used for consumer electronics, such as portable computers, tablets and mobile phones.
However, capacitive fingerprint sensing devices are typically sensitive to contaminants located between the capacitive sensing elements of the sensor and the finger to be measured. Contaminants may be introduced from the ambient, such as e.g. moisture from the air, or from the surface of a person's fingertip (such as e.g sweat, hand lotion etc). For example, residual fingerprints on the sensing surface may reduce the accuracy of the sensing device, and may in worst case lead to erroneous readings. Contaminants on the sensing surface located such that they span across boundaries between sensing elements may for example lead to a coupling between adjacent sensing elements, which in turn reduces the accuracy of the sensing device.
Accordingly, it is desirable to reduce the occurrence of contaminants on the sensing surface of a capacitive fingerprint sensing device
To address the presence of contaminants, U.S. Pat. No. 7,071,708 discloses a chip type sensor, which may be a capacitive fingerprint sensor, having a protection layer comprising a polymeric material or ceramic atomic layer to provide a hydrophobic and lipophobic surface. The purpose of the protective layer is to prevent a latent fingerprint from being formed on the surface.
However, materials which are hydrophobic and/or lipophobic, also referred to as easy-to-clean materials, are typically sensitive to mechanical abrasion and daily wear and tear. Under the influence of abrasive action, they are either physically removed or lose the easy-to-clean properties, which chemically may only be attributed to the very few first nanometers of the surface. This imposes problems of utilizing such materials for fingerprint sensors, since the easy-to-clean properties will degrade with normal usage of the sensor over time. This is particularly problematic for swipe sensors where the user is required to slide the fingertip over the sensor surface.